Bump version to 21.0.0 #3939

Job	Run time
rust-analyzer-compat	2m 18s
fmt	7s
build-fuzz	48s
build-and-test (msrv, ubuntu-latest, x86_64-unknown-linux-gnu)	4m 41s
cargo-deny (advisories)	38s
build-and-test (msrv, ubuntu-latest, aarch64-unknown-linux-gnu, true)	2m 13s
cargo-deny (bans licenses sources)	40s
build-and-test (msrv, macos-latest, x86_64-apple-darwin)	30m 16s
docs	2m 11s
build-and-test (msrv, macos-latest, aarch64-apple-darwin)	19m 21s
check-git-rev-deps	4s
build-and-test (msrv, windows-latest, x86_64-pc-windows-msvc)	14m 18s
readme	31s
build-and-test (latest, ubuntu-latest, x86_64-unknown-linux-gnu)	4m 7s
semver-checks	3m 59s
build-and-test (latest, ubuntu-latest, aarch64-unknown-linux-gnu, true)	2m 3s
build-and-test (latest, macos-latest, x86_64-apple-darwin)	28m 39s
build-and-test (latest, macos-latest, aarch64-apple-darwin)	9m 44s
build-and-test (latest, windows-latest, x86_64-pc-windows-msvc)	13m 29s
publish-dry-run (ubuntu-latest, wasm32-unknown-unknown) / publish-dry-run	2m 30s
publish-dry-run (ubuntu-latest, x86_64-unknown-linux-gnu, --feature-powerset --exclude-features d... / publish-dry-run	4m 26s
complete	1s
	2h 27m 4s

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